Fresh concerns about Lord Ashcroft emerged tonight when he was accused of "systematic tax avoidance" by exploiting his offshore status to avoid paying VAT on opinion polls he commissioned for the Conservatives.

Ashcroft privately ordered what he boasted was the biggest political polling exercise ever conducted in Britain in 2005, in order to aid the Tories as they targeted marginal seats. The cost of the polls, commissioned from YouGov and Populus, is believed to have approached at least £250,000.

But sources familiar with the transactions told the Guardian that the bills were paid by one his companies in Belize, meaning he did not pay VAT.

Tonight, the Liberal Democrats' Treasury spokesman, Vince Cable, said: "This is quite serious. We are now not talking just about Ashcroft's non-dom status, but about systematic tax avoidance in funding Conservative party activities such as polling. How far were the Conservatives aware that Ashcroft did not pay VAT, as would have been incurred by any normal polling activity?"

The new allegations came amid growing concerns in David Cameron's circle over the handling of the affair by William Hague, his shadow foreign secretary.

It emerged earlier today Hague kept Cameron in the dark for at least a month after he learned that Ashcroft had renegotiated the terms of his peerage and acquired non-dom status. Cameron found out the truth about Ashcroft's tax affairs less than a month ago. A party spokesman confirmed that Ashcroft did not even reveal his tax status to Cameron when in December the leader approached him to discuss plans to ban non-doms from parliament, despite the fact that he had already told Hague.

The Tories also became embroiled in a row with the Electoral Commission after its official inquiry into Ashcroft's donations via his company, Bearwood Corporate Services, found it had not breached any donor rules but criticised party officials for refusing to give evidence in person.

Discussing the Guardian's VAT revelations, polling company sources said a single poll of a sample of 2,000 people typically cost £20,000 to £25,000. Ashcroft not only commissioned a series of tracking polls day-by-day in the run-up to the 2005 election, but used enormous samples of up to 10,000. One pollster said: "Such polling projects in the commercial sector frequently cost more than £250,000." This means that VAT in excess of £40,000 could have been avoided.

At the time, Ashcroft was resident in Britain and depicted himself as having paid for the polling personally.

One source said instructions had been sent by Ashcroft to the polling companies to send invoices on the basis they were "export" orders from outside the EU, and thus not to charge VAT. "It was invoiced to Belize and therefore didn't attract VAT," the source said.

Ashcroft subsequently published his detailed results in a book called Smell the Coffee: a Wake-up Call for the Conservative Party, as a result of which Cameron gave him the influential position of deputy party chairman, in December 2005.

He said in the introduction to his book: "A research programme of this scale has been an enormous undertaking. The expert pollsters from Populus and YouGov have been extraordinarily professional and great fun to work with."

His findings, which strongly influenced Cameron's subsequent tactics, were that most people believed the Tories to be "out of touch", "opportunistic", "don't care about ordinary people", "stuck in the past", and "care more about the well-off than the have-nots".

The sums of VAT saved by a manoeuvre that was not illegal, were relatively small to a multimillionaire like Ashcroft.

But he explained his business approach in his own memoirs called "Dirty Politics, Dirty Times" when he describes selling doughnuts to fellow schoolboys at an undisclosed profit: "There were probably people then as now who – if they discovered exactly what I was doing – might have found my practice a little sharp.

"I looked upon it as simply working to find an edge, the sort of advantage I would search for time and again."

Ashcroft's spokesman declined to comment today on the allegations that the wealthy businessman had not paid VAT to the polling companies, and that he had spent at least £250,000 on the projects. Neither YouGov nor Populus was prepared to comment .

Tonight a Conservative spokesman disavowed responsibility for Ashcroft's tactics, saying: "We do not recognise this as Conservative party polling."